My coral is angry?

[Peterupnorth]

Really enjoy reading this forum. I'm about 8 months new at this, and going through the very steep learning curve.
My daughter brought me a green plate coral and it wasn't expanding near as much as what she saw it in the store. So I put it on top of a rock, thinking maybe more current and closer to the light would help. A few hours later just happened to see it spit out a rather large cloud of something whitish; looked like a mushroom cloud in the tank.
I immediately looked it up on a couple sites and found out that I shouldn't put it on a rock!!! It's back in the gravel and seems relatively OK but seen it spit out again. What's it doing??
I have been adding "reef builder".
Peter.

35 gal tank, 2 clowns, 2 damsels, emerald crab, frogspawn, generally everything looks fine.